1. Field of the Invention
The present invention relates in general to a shift-register circuit. In particular, the present invention relates to a shift-register circuit for liquid crystal display.
2. Description of the Related Art
FIG. 1 shows a conventional shift-register circuit as disclosed by U.S. Pat. No. 5,434,899 of Huq in 1995. FIG. 1 only shows a single shift-register unit, although a plurality of shift-register units compose a shift-register circuit. The input signal from the input terminal INPUT switches the NMOS transistor 12 through the NMOS transistor 10. The output signal is at low voltage level when the input signal is at high voltage level and the clock signal is at low voltage level. Thus, the capacitor 14 is charged, and a voltage difference is generated on both sides of the capacitor 14. However, the turned on NMOS transistor 10 acts as a diode, which lowers the voltage level of the signal controlling the NMOS transistor 12.
The voltage level of the output signal is lowered when the NMOS transistor 16 is turned on. The NMOS transistor 16 is turned off and the input clock C1 starts to provide high-voltage level signal when the voltage difference of the capacitor 14 is higher than the threshold voltage of the NMOS transistor 12. Thus, the output terminal outputs high-voltage level signal to the next-stage shift-register unit and the output signal returns to low-voltage level when the NMOS transistor 16 is turned on again.
In addition, the NMOS transistor 18 is switched by the output signal of the next two-stage shift-register unit. When the NMOS transistor 18 is turned on, the voltage difference of the capacitor 14 rapidly decreases. Thus, the NMOS transistor 12 is turned off and the output terminal stops outputting the signal.
However, the circuit layout of the conventional shift-register circuit is complicated because feedback circuit is required. In addition, the circuit layout of the conventional shift-register circuit is more complicated because the feedback signal is provided by the next two-stage shift-register unit. In addition, the input signal is provided through the turned on NMOS transistor acting as a diode, so the voltage level of the signal is decreased and the tolerance of the conventional shift-register circuit deteriorates.